There isn't a specific symbol used to represent "heat needed" in a decomposition reaction. Instead, we use the following conventions:

* "Δ" (delta) over the arrow: This signifies that heat is added to the reaction. It's a general symbol for heat input, not specific to decomposition reactions.

* "Heat" or "Heat energy" above the arrow: This is a more descriptive way to indicate heat is added.

Example:

* CaCO3 (s) + Δ → CaO (s) + CO2 (g) This shows the decomposition of calcium carbonate (CaCO3) into calcium oxide (CaO) and carbon dioxide (CO2) with the addition of heat.

Remember that decomposition reactions are endothermic, meaning they require energy (usually in the form of heat) to occur.